Electrostatic developing apparatus



May 19, 1964 M. M. SOWIAK ELECTROSTATIC DEVELOPING APPARATUS Filed June22, 1961 NJWN H. v 0 01 LA INVENTOR. 411712 1 M Jon 721k 4rmewir UnitedStates PatentOfilice 3,133,834 Patented May 19, 1964 3,133,834ELECTRGSTATIC DEVELOPING APPARATUS Milton M. Sowiak, Mercerville, N.J.,assignor to Radio Corporation oi America, a corporation of Delaware June22, 1%1, Ser. No. 118,920 8 Claims. (Cl. 118-637) This invention relatesgenerally to electrostatic printing, and more particularly to improvedapparatus for developing an electrostatic latent image on anelectrophotographic recording element by means of an electroscopicdeveloper mixture of magnetic particles and toner particles. Theimproved apparatus of the present invention is particularly useful fordeveloping images on a recording element in the form of a web moving ata relatively ln'gh speed.

It has been proposed to develop latent electrostatic images on arecording element in the form of a web by drawing the web through anelectroscopic developer mixture of magnetic particles and tonerparticles. At relatively slow speeds of travel, this method ofdevelopment is quite satisfactory. At relatively higher speed, however,the actual developing time is decreased and there is a relativelygreater tendency for the magnetic particles of the developer mixture toadhere to the recording element. This is especially true when theambient humidity is low. To provide satisfactory development of latentelectrostatic images on a recording element moving at a relatively highspeed, therefore, means must be provided to remove unwanted magneticparticles from the recording element.

Accordingly, it is an object of the present invention to provideimproved apparatus for developing latent electrostatic images on arecording element moving at a relatively high speed.

Another object of the present invention is to provide improved apparatusfor removing magnetic particles of an electroscopic developer mixturefrom a recording element in an electrostatic printing system wherein therecording element is moved at a relatively high speed.

A further object of the present invention is to provide improvedapparatus of the type described that is relatively simple in structure,easy to operate, and highly efficient in use.

In accordance with the present invention, the improved developingapparatus comprises three drums, each one being slightly spaced from theother two and each one having its axis disposed parallel to the axes ofthe other two. The three drums are disposed within a trough-likecontainer for an electroscopic developer mixture of magnetic particlesand toner particles. One of the drums, a mixing drum, is made ofmagnetic material and is connected to means to cause it to rotate in thedeveloper mixture to stir the latter. Another of the drums, a tonerfeeddrum, is made of non-rnagnetic material. A magnet is fixedly disposedwithin the toner-feed drum, and means are provided to cause the drum torotate about its axis, the magnet remaining stationary. The third drum,an element-guiding drum, may comprise only a portion of a drum, andserves to guide the recording element along the path for properdevelopment. A pole piece is fixedly disposed within the element-guidingdrum in a manner to form a magnetic circuit with the aforementionedmagnet and the mixing drum. The gaps between the drums are paths in themagnetic circuit. Magnetic particles and toner particles adhering to themagnetic particles tend to be concentrated in the gaps between thedrums. The recording element, in contact with the element-guiding drum,is moved through the gaps so that the toner particles develop the latentelectrostatic image previously established thereon. \Nith thisarrangement, the latent electrostatic image is developed in two gapswherein strong magnetic fields cause the magnetic particles to act asstill brushes, thereby reducing the possibility of magnetic particlesadhering to the recording element.

Other novel features of the present invention, as well as additionalobjects and advantages thereof, will be described in greater detail withreference to the accompanying drawing, the single figure of which ispartly schematic, cross-sectional side elevational view of thedeveloping apparatus of the present invention.

Referring now to the drawing, there is shown developing apparatus 10 fordeveloping a latent electrostatic image on a recording element 12. Therecording element 12 may comprise a web of relatively conductivematerial, such as paper having a predetermined moisture content, andcoated with an insulating layer 14, on which the latent electrostaticimage may be formed by any suitable means known in the art. An exampleof a suitable recording element 12 is described in RCA Review, December1954, Volume XV, No. 4.

The developing apparatus ill comprises a trough-like non-magneticcontainer 16 for holding a quantity of an electroscopic developermixture 18 of magnetic particles and toner particles. The developermixture 18 exhibits a tribo-electric effect in that the magneticparticles and the toner particles acquire electric charges of oppositepolarity when they are separated from each other after having beenstirred together. It is the charged toner powder of the developermixture that develops the latent electro static image as willhereinafter be explained.

Means are provided to stir the developer mixture 18 to provide arelatively uniform mixture of magnetic particles and toner particles andalso to enhance the triooelectric effect. To this end, a mixing drum 2%is mounted for rotation about its axis, counterclockwise as viewed inthe drawing as within the container 16. The mixing drum 20 is disposedwithin the developer mixture 13, and a motor 22, is coupled to the shaft24 of the drum 20 to rotate the latter. The mixing drum 20 is made ofmagnetic material and a plurality of nonmagnetic blades 26 are fixed tothe drum 24 to increase the efliciency of the drums stirring action.

A toner-feed drum 3!) is also disposed within the container 16. Thetoner-feed drum St? is made of non-magnetic material and has an outerknurled surface 31. The toner-feed drum 30 is slightly spaced from themixing drum 20 and has its axis substantially parallel to the axis ofthe mixing drum 20. The toner-feed drum 30 is coupled to the motor 22for rotation about its axis, by any suitable means (not shown).

A magnet 32, such as an electromagnet, is fixedly disposed within thetoner-feed drum 3% so that it remains stationary when the toner-feeddrum 3t) revolves. The magnet 32 has two pole pieces, an upper polepiece 34 and a lower pole piece 36, and the axis between them is transverse to the axis of rotation of the toner-feed drum 30.

A toner-feed trough 38 is disposed over the toner-feed drum 30 forsupplying toner particles it) to the developer mixture 18 to replace thetoner particles that are removed from the developer mixture 18 duringthe developing process. The trough 38 is hinged to a bracket 42. A wingscrew 44, threaded in the bracket 42 and captivated in a wall of thetrough 38, provides means to adjust the trough 38 with respect to thetoner-feed drum 30. A cloth material, such as plush 46, is disposedbetween the lower edges of the trough 38 and the knurled surface of thetoner-feed drum 30. Thus, toner particles 40 caught in the grooves ofthe knurled surface of the tonerfeed drum 30 are conveyed to thedeveloper mixture 18 as the toner-feed drum 30 is rotated.

The toner-feed drum 30 is caused to rotate clockwise in the directionindicated by the arrow 48, that is, in a direction opposite to thedirection of rotation of the mixing drum 20. The speeds of rotation ofthe drums 20 and 30 may be adjusted by any suitable means (not shown)and may be different from each other, as will be hereinafter explained.

The recording element 12 is guided adjacent to the surfaces of themixing drum 20 and the toner-feed drum 30 by means of an element-guidingdrum 50 of non-magnetic material. The element-guiding drum 50 is mountedfor free rotation around its axis, the latter being substantiallyparallel to the axes of the drums 20 and 30. Thus, a continuous web ofthe recording element 12 can be moved in the directions indicated by thearrows 52 and 53 by pulling the recording element 12 to the left asviewed in the drawing, so that the friction between the drum 50 and therecording element 12 causes the drum to rotate clockwise as indicated bythe arrow 54. A magnetic pole piece 56 is fixedly disposed within theelementguiding drum 50. One end of the pole piece 56 is adjacent to thepole piece 34 of the magnet 32, and the other end is adjacent to themixing drum 20. The pole piece 56 is fixed to a fixed shaft 58 by meansof a bar 59.

The drums 20, 30, and 50 are slightly spaced from each other. A gap Abetween the drums 20 and 50 is larger than a gap B between the drum 30and the drum 50, but is smaller than a gap C between the drum 30 and thedrum 20. A magnetic circuit, indicated by the dashed closed path 60, isformed by the upper pole piece 34 of the magnet 32, the pole piece 56, aportion of the mixing drum 20, and the lower pole piece 36 of the magnet32. The gaps A, B, and C also form portions of the magnetic circuit path60. During the developing process, as will be hereinafter explained,magnetic particles and toner particles adhering to the magneticparticles form a brush-like structure in the gaps A and B for brushingthe toner particles onto the latent electrostatic image on the recordingelement 12 to develop the latter. The lengths of the gaps A, B, and Cmay be for example, in., in., and A3 in., respectively.

A magnet 62, such as an electromagnet, is disposed underneath thecontainer 16, by means of bracket 63, just below the point where therecording element 12 comes in contact with the element-guiding drum 50.The magnet 62 comprises pole pieces 64 and 66 to provide a magnetic path65 between the lower wall of the container 16 and the photosensitivelayer 14 of the incoming recording element 12. Developer mixture 18 isheld in a brush-like manner by the magnetic field 65 so that the latentelectrostatic image on the recording element 12 is wiped with thedeveloper mixture 18 as the recording element 12 enters the container16. The developer in the magnetic field 65 not only provides apreliminary developing action, but also serves as a seal to prevent lossof the developer mixture from the container 16.

The developer mixture 8 may be removed from the container 16 through anopening 70 in the lowermost wall of the container 16. The opening 70 iscovered with a cover 72 that is retained in place by means of a springlatch 74. One end of the spring latch 74 is riveted to the lower wall ofthe container 16, and the other end of the spring latch 74 is releasablyretained in a spring lock 76. A baffle plate 78 is disposed within thecontainer 16 to cause the developer mixture to gravitate toward themixing drum 20 and to prevent the developer mixture 18 from beingretained in dead space.

The operation of the improved developing apparatus will now bedescribed: The recording element 12 is moved to the left in thedirection of the arrows 52 and 53 so as to cause the element-guidingdrum 50 to rotate clockwise in the direction of the arrow 54. Theinsulating layer 14 of the recording element 12 is the remote surface ofthe recording element 12 with respect to the element guiding drum 50. Asthe latent image on the insulating layer 14 passes the magnetic field65, toner particles are brushed onto the layer 14 and partialdevelopment of the latent electrostatic image results. The mixing drum20 is rotated counterclockwise so that its peripheral speed is differentfrom that of the drum 50. Thus, as the recording element 12 passes thegap A, stiff, bushlike, magnetic particles produce a wiping action onthe insulating layer 14, thereby continuing the developing action. Anymagnetic particles that are retained on the recording element 12 arepicked up in the gap B between the drums 30 and 50 as the recordingelement 12 is moved. The direction of rotation of the drum 30 is such asto brush the magnetic particles back into the bottom of the container16. Toner particles 40 from the trough 38 are also brought into the gapB so that they can mix with any magnetic particles there and thusprovide further developing action of the latent electrostatic image. Thedeveloper mixture 18 is prevented from clogging the gaps A, B, and Cbecause each of the three drums 20, 30 and 50 is rotated. Thus it willbe seen that the recording element 12 encounters two stifi magneticbrushes in the air gaps A and B. Since these magnetic brushes are formedin the concentrated magnetic flux of the magnetic path 60, magneticparticles tend to remain in the'magnetic path rather than move with therecording element 12.

The blades 26 on the mixing drum 20 are of non-magnetic material so thatthey do not distort the magnetic field and the magnetic brush formed inthe gap A as the drum is rotated. A distorted magnetic field may resultin non-uniform development of the recording element 12. The toner-feeddrum 30 should be rotated at a speed at which a good magnetic brush ismaintained in the gap B and at which an equilibrium condition isobtained wherein the quantity of magnetic particles removed from the gapB is equal to the quantity of magnetic particles brought into the gap Bby the recording element 12. The gap C is slightly larger than the gap Aso that it is not possible for more developing mixture 18 to enter thespace between the three drums than is leaving. The developer mixture 18emerging from the gap C eventually falls into the bottom of thecontainer 16.

The pressure of the toner-feed trough 38 against a knurled surface ofthe toner-feed drum 30 is adjusted so as to provide a state ofequilibrium between the amount of toner particles 40 admitted into thecontainer 16 and the amount of toner particles removed by thedevelopment of the latent electrostatic images on the recording element12. Since the magnetic particles of the developer mixture 18 areconstantly being returned to the developer mixture, no special provisionneed be made to replace magnetic particles in the developer mixture 18.

It is within the contemplation of the present invention that theelement-guiding drum 50 be fixed so that the recording element 12 may bepulled past the gaps A and B Without the drum 50 rotating. Under thelatter conditions, the element-guiding drum 50 need not even comprise acomplete drum, but may comprise only that portion of the drum necessaryto guide the recording element 12 past the gaps A and B. As used herein,therefore, and in the appended claims, the term element-guiding drum maycomprise a complete drum or only that portion of a drum (which may bestationary) necessary to guide the recording element past the gaps wheredevelopment takes place.

From the foregoing description, it will be apparent that there has beenprovided improved apparatus for developing an electrostatic latent imageon a recording element. By means of the apparatus described, therecord-ing element receives most of its development in a gap between amixing drum and an element-guiding drum, the gap being a portion of apath in a strong magnetic circuit. Substantially any magnetic particlesthat adhere to the recording element after it is developed in the firstgap are removed by a brush of magnetic particles [in another gap of themagnetic circuit between a toner-feed drum and the element-guiding drum.The direction of rotation of the toner-feed drum is such as to brush themagnetic particles back into the developer mixture. The

mixing drum and the toner-feed drum are rotated at different speeds fromthe speed of rotation of the elementguiding drum to provide a Wipingaction on the recording element. For example, the mixing drum may berotated at a speed of 60 revolutions per minute, the toner-feed drum maybe rotated at 50 revolutions per minute, and the element-guiding drummay be rotated at 90 revolutions per minute in a typical case. Also, thedeveloping apparatus of the present invention may be used for directdeveloping or reversal developing with toners of the type described inUS. Patent 2,874,063, for Electrostatic Printing, issued February 17,1959, and in "US. Patent 2,986,- 521, for Reversal Type ElectrostaticDeveloper Powder, issued May 30 1961, respectively. While the apparatushas been illustrated partly in schematic form, variations in theapparatus coming within the spirit of this invention will, no doubt,readily suggest themselves to those skilled in the art. Hence, it isdesired that the foregoing description shall be considered merely asillustrative and not in a limiting sense.

What is claimed is:

1. Apparatus comprising a container for a developer mixture of magneticparticles and toner particles, a first drum of magnetic material, asecond drum and a third drum both of non-magnetic material, at leastportions of said drums being disposed within said container, each ofsaid drums being spaced slightly from the other of said drums andforming gaps therebetween, each of said drums having an axissubstantially parallel to the axes of said other of said drums, meanscoupled to said first drum and to said second drum to rotate them inopposite directions, said first drum being adapted to stir saiddeveloper mixture when it is rotated, and said third drum being adaptedto support a web recording element for movement through the gap betweensaid third drum and said first drum and the gap between said second drumand said third drum and magnetic means fixedly disposed within saidsecond drum and within said third drum and forming a magnetic circuitwith a portion of said first drum, said gaps comprising portions of thepath of said magnetic circuit, the elements forming said magneticcircuit cooperating to produce a magnetic brush of magnetic and tonerparticles in the gap between said first and second drums and in the gapbetween said second and third drums.

2. Apparatus comprising a container for a developer mixture of magneticparticles and toner particles, a mixing drum, a toner-feed drum, and anelement-guiding drum, at least portions of said drums being disposedwithin said container, each of said drums being spaced slightly from theother of said drums the forming gaps -there between and having an axissubstantially parallel to the axes of said other of said drums, saidelement guiding drum being adapted to support a web recording elementfor movement through the gap between said element guiding drum and saidmixing drum and the gap between said element guiding drum and saidtoner-feed drum, means coupled to said mixing drum and to said tonerfeeddrum to rotate said mixing drum and said tonerfeed drum in oppositedirections, said mixing drum comprising magnetic material, saidtonerfeed drum and said element-guiding drum comprising non-magneticmaterial, and magnetic means fixedly disposed within said tonerfeed drumand within said element-guiding drum, said magnetic means forming amagnetic circuit with a portion of said mixing drum, the elementsforming said magnetic circuit cooperating to produce a magnetic brush ofmagnetic and toner particles in the gap between said element guidingdrum and said toner feed drum.

3. Apparatus comprising a container for a developer mixture of magneticparticles and toner pmticles, a mixing drum of magnetic material, atonerfeed drum and an element-guiding drum both of non-magneticmaterial, at least portions of said drums being disposed within saidcontainer, each of said drums being spaced slightly from the other ofsaid drums and forming gaps therebetween, each of said drums having anaxis substantially parallel to the axes of said other of said drums,means coupled to said mixing drum and to said tonerfeed drum to rotatesaid mixing drum and said tonerteed drum in directions opposite to eachother, said element guiding drum being adapted to support a webrecording element for movement through the gap between said elementguiding drum and said mixing drum and the gap between said elementguiding drum and said toner-feed drum, and magnetic means fixedlydisposed within said toner-feed drum and within said element-guid-in gdrum and forming a magnetic circuit with a portion of said mixing drum,the elements forming said magnetic circuit cooperating to produce amagnetic brush of magnetic and toner particles in the gap between saidelement guiding drum and said mixing drum and in the gap between saidelement guiding drum and said toner-feed drum.

4. Developing apparatus comprising a container for a developer mixtureof magnetic particles and toner particles, a mixing drum, a toner-feeddrum, and an elementguiding drum, at least portions of said drums beingdisposed within said container, each of said drums being slightly spacedfrom the other of said drums and forming gaps therebetween, each of saiddrums having an axis substantially parallel to the axis of said other ofsaid drums, means coupled to said mixing drum and said toner-feed drumto rotate them in directions opposite to each other, said mixing drumcomprising magnetic material and being adapted to stir said developermixture when rotated, said toner-teed drum and said element-guiding dmmcomprising non-magnetic material, and magnetic means fixedly disposedwithin said toner-feed drum and within said element-guiding drum andforming a magnetic circuit with a portion of said mixing drum and withsaid gaps, said element-guiding drum comprising means to support a webrecording element for movement through the gap between said elementguiding drum and said mixing drum and the gap between said elementguiding drum and said toner-feed drum, the gap between said mixing drumand said element-guiding drum being larger than the gap between saidtonerfeed drum and said element-guiding drum, the elements forming saidmagnetic circuit cooperating to produce a magnetic brush of magnetic andtoner particles in the gap between said element guiding drum and saidmixing drum and in the gap between said element guiding drum and saidtoner-feed drum.

5. Apparatus for developing a latent electrostatic image on a webrecording element by means of an electroscopic developer mixture ofmagnetic particles and toner particles, said apparatus comprising acontainer for said mixture, a mixing drum of magnetic material disposedwithin said container, means connected to said mixing drum to rotate itabout its axis whereby to stir said mixture, a second drum ofnon-magnetic material spaced from said mixing drum and disposedsubstantially parallel thereto, a magnet fixedly disposed within saidsecond drum and having one or" its poles adjacent to said mixing drum,means to rotate said second drum in an opposite direction to thedirection of rotation of said mixing drum, a third drum mounted forrotation adjacent to said mixing drum and to said second drum anddisposed substantially parallel thereto, and a pole piece fixed withinsaid third drum and having one end disposed adjacent to said mixing drumand another end adjacent to the other of said poles of said magnet, saidthird drum being spaced from said mixing drum and from said second drumand storming gaps therebetween, said third drum comprising means tosupport said web recording element for movement through the gap betweensaid third drum and said mixing drum and the gap between said third drumand said second drum, said gaps comprising a portion of the path or amagnetic circuit formed with said magnet, said pole piece, and saidmixing drum, the elements forming .said magnetic circuit cooperating toproduce a magnetic brush and magnetic and toper particles in the gapbetween said third drum and said mixing drum and in the .gap betweensaid third drum and said second drum.

6. In an electrostatic printing system of the type wherein a latentelectrostatic image on a web recording ele- .ment is developed withtoner particles from an electrodirections, said third drum being adaptedto support said web recording element for movement through the gapbetween said first and third drums and the gap between said second andthird drums, the elements forming said magnetic circuit cooperating toproduce a magnetic brush of said electroscopic mixture in the gapbetween said first and third drums and the gap between said second andthird drums.

7. In an electrostatic printing system of the type wherein a latentelectrostatic image on a web recording element is developed with tonerparticles from an electroscopic mixture including magnetic particles,improved developing apparatus comprising three drums each of said drumsbeing spaced slightly from each other and forming gaps therebetween,each of said drums having an axis substantially parallel to the axes ofthe other of said drums, the first of said drums comprising magneticmaterial, magnetic means fixedly disposed within each of the second andthird of said drums and forming a magnetic circuit with a portion ofsaid first drum, said gaps comprising portions of said magnetic circuit,and said first drum being adapted to be disposed in said mixture andhaving non-magnetic mixing means fixed thereto, and

means'to rotate said first and second drums in opposite directions, thethird of said drums being adapted to support said web recording elementfor movement through the gap between said first and third drums and thegap between said second and third drums, the elements forming saidmagnetic circuit cooperating to produce a magnetic brush of saidelectroscopic mixture in the gap between said first and third drums andin the gap between said second and third drums.

8. In combination, a container for a developer mixture of magneticparticles and toner particles, a mixing drum .of magnetic materialdisposed within said container and adapted to stir said developermixture when rotated, a toner-feed drum disposed substantially parallelto said mixing drum and spaced therefrom, an element-guiding drumdisposed substantially parallel to both said mixing drum and saidtoner-feed drum and spaced therefrom, said element-guiding drum beingadapted to support a Web recording element for movement through thespace between said element-guiding drum and said mixing drum and thespace between said element-guiding drum and said toner-feed drum,magnetic means fixedly mounted within said toner-feed drum and Withinsaid element guiding drum and forming a magnetic circuit with a portionof said mixing drum, the elements forming said magnetic circuitscooperating to produce a magnetic brush of said developer mixture in thespace between said element-guiding drum and said mixing drum and in thespace between said element-guiding drum and said toner-feed drum, andmeans for rotating said toner-feed drum and said mixing drum in oppositedirections, said element-guiding drum being adapted to rotate freely.

References Cited in the file of this patent UNITED STATES PATENTS1,714,171 Jobke -May 21, 1929 2,732,775 Young et a1. Jan. 31, 19562,786,440 Giaimo Mar. 26, 1957 2,826,168 Grant ...Q Mar. 11, 19583,003,462. Streich Oct. 10, 1961

1. APPARATUS COMPRISING A CONTAINER FOR A DEVELOPER MIXTURE OF MAGNETICPARTICLES AND TONER PARTICLES, A FIRST DRUM OF MAGNETIC MATERIAL, ASECOND DRUM AND A THIRD DRUM BOTH OF NON-MAGNETIC MATERIAL, AT LEASTPORTIONS OF SAID DRUMS BEING DISPOSED WITHIN SAID CONTAINER, EACH OFSAID DRUMS BEING SPACED SLIGHTLY FROM THE OTHER OF SAID DRUMS ANDFORMING GAPS THEREBETWEEN, EACH OF SAID DRUMS HAVING AN AXISSUBSTANTIALLY PARALLEL TO THE AXES OF SAID OTHER OF SAID DRUMS, MEANSCOUPLED TO SAID FIRST DRUM AND TO SAID SECOND DRUM TO ROTATE THEM INOPPOSITEDIRECTIONS, SAID FIRST DRUM BEING ADAPTED TO STIR SAID DEVELOPERMIXTURE WHEN IT IS ROTATED, AND SAID THIRD DRUM BEING ADAPTED TO SUPPORTA WEB RECORDING ELEMENT FOR MOVEMENT THROUGH THE GAP BETWEEN SAID THIRDDRUM AND SAID FIRST DRUM AND THE GAP BETWEEN SAID SECOND DRUM AND SAIDTHIRD DRUM AND MAGNETIC MEANS FIXEDLY DISPOSED WITHIN SAID SECOND DRUMAND WITHIN SAID THIRD DRUM AND FORMING A MAGNETIC CIRCUIT WITH A PORTIONOF SAID FIRST DRUM, SAID GAPS COMPRISING PORTIONS OF THE PATH OF SAIDMAGNETIC CIRCUIT, THE ELEMENTS FORMING SAID MAGNETIC CIRCUIT COOPERATINGTO PRODUCE A MAGENTIC BRUSH OF MAGNETIC AND TONER PARTICLES IN THE GAPBETWEEN SAID FIRST AND SECOND DRUMS AND IN THE GAP BETWEEN SAID SECONDAND THIRD DRUMS.